There are numerous applications where it is desirable to provide an aluminum film or coating on glass. These include light reflecting films, such as on mirrors and lamp envelopes, and electrically conducting films.
Such aluminum films function quite satisfactorily at ambient, or low, temperatures. However, at elevated temperatures, there is a strong tendency for aluminum to react with conventional, silicate and phosphate glasses to form the oxide, Al.sub.2 O.sub.3. In silicate and phosphate glasses, the SiO.sub.2 and P.sub.2 O.sub.5 are reduced to the elemental state. This, of course, is usually quite unacceptable.
Efforts to dissolve aluminum in a conventional glass, and to precipitate it out as colloidal metal, were also unsuccessful. The metal became oxidized, and incorporated in the glass composition, during batch melting.
The present invention is based on the discovery that a family of glasses is compatible with, and does not react with, aluminum, even at elevated temperatures. Accordingly, a basic purpose of the invention is to provide such family of aluminum-compatible glasses.
Another purpose is to provide glasses in which aluminum can be dissolved, and then precipitated out in colloidal form.
A further purpose is to provide a novel, polarizing glass utilizing a glass containing precipitated, aluminum metal particles.